1. Sauté the onion over a medium low heat in a large soup pot until lightly coloured (2-3 minutes). Add the garlic and stir for 30 seconds. Add 1/4 cup of the stock and stir to deglaze the pot.
2. Add the rest of the stock, tomatoes, purée, sweetener, vinegar (if using), spinach, beans, and the 1/2 tsp black pepper.
3. Bring to the boil. Reduce the heat, cover, and simmer for 5 minutes.
4. Add the chopped basil and season with black pepper to taste.